1. Field of the Invention
The present invention relates to a device for measuring extrasurface shape and intrasurface shape by scanning with a laser beam.
2. Description of the Related Art
It has been increasingly urged to measure the extrasurface shape which is a change in the direction of height of a precisely machined sample maintaining a precision of 1 nm, as well as to measure the intrasurface shape such as the size and shape of a pattern or configuration of areas formed by different materials, respectively, formed on the surface, maintaining a precision of 10 nm. A method which utilizes optical interference has been employed for measuring extrasurface shapes such as surface coarseness and the like, and an optical heterodyne interference method is effective in highly precise measuring of a very small change in height. This method consists of forming beat signals of a differential frequency by interference between two laser beams of different frequencies, and detecting a change in the phase of beat signals maintaining a resolution of about 1/500 wavelength to measure a change of the surface in the height direction. This change in the phase corresponds to a difference in the length of optical path between the two beams. Among the optical heterodyne interference methods, a differential heterodyne interference method invented by the present inventors has been described in Japanese Patent Publication No. 44243/1991 entitled "Device for Measuring Surface Shape by the Optical Heterodyne Interference Method", according to which two beams of different frequencies are generated by driving an acousto-optical element with electric signals of two frequency components in order to detect a change in phase between the two beams.
For measuring the intrasurface shapes, there has been widely employed a method according to which the surface is scanned with a laser beam which is focused into a tiny spot, and a change in the intensity of light reflected from the sample is detected. When the surface is constituted by a plurality of members having different reflection factors, the intensity of the reflected light undergoes a change depending upon the distribution of reflection factors. A change in the intensity of the reflected light caused by the change in the reflection factor is calculated to detect an edge which is a boundary at which the reflection factor changes, and the size and shape of the pattern are measured from a change in the edge position. In particular, a laser scanning-type confocal microscope has been used in a variety of fields since it is capable of obtaining inner surface resolution greater than that of ordinary microscopes. This method consists of detecting (confocal detecting) the light reflected from the sample through a pinhole, and the scattering light that becomes noise is cut off in order to increase the inner surface resolution. Moreover, the confocal microscope is capable of measuring a change in the height in the direction of the focal point since it detects the intensity of light reflected from the position of focal point of the spot beam at which it falls on the sample. In this case, the sample is moved by a pulse stage or the like in the direction of the optical axis, and the data related to the intensity of the reflected light detected at each of the positions are processed in order to measure the intrasurface and extrasurface three-dimensional shapes.
However, though the above-mentioned laser scanning-type confocal microscope is capable of measuring the intrasurface and extrasurface shapes, the sample must be moved by a mechanical stage when the extrasurface shape is to be measured. Moreover, since the focal depth of the spot light falling on the sample is relatively shape is about 0.1 .mu.m. With the laser scanning-type great, the resolution for measuring the extrasurface shape is about 0.1 .mu.m. With the laser scanning-type cofocol microscope, therefore, it is not allowed to measure a change in the extrasurface shape which is as small as about 10 nm. The optical heterodyne interference makes it possible to measure a change in the extrasurface shape of about several nm but does not make it possible to measure the intrasurface shape.
That is, the phase detection based on the above-mentioned conventional optical heterodyne interference method makes it possible to detect a change in the direction of height on the extrasurface but does not make it possible to detect a change in the intrasurface shape. This is because, the phase date include only the data related to the lengths of optical paths in the direction of height. With the method of detecting a change in the intensity of the reflected light, on the other hand, a change in the intrasurface shape can be detected but a change in the extrasurface shape cannot be detected. This is because, when a change in the direction of height of the surface is within the focal depth of the irradiated light, the intensity of the reflected light remains constant with respect to a change in the direction of height. Therefore, it is not possible to simultaneously measure by using one measuring device the extrasurface shape and the intrasurface shape of a sample that has a shape changing on the extrasurface and on the intrasurface. That is, separate measuring devices have been used depending on the purpose of measurement. It is therefore an object of the present invention to realize a measuring device of a novel constitution which is capable of simultaneously measuring the intrasurface shape and the extrasurface shape maintaining high precision by solving the aforementioned problems.